1. Field of the Invention
This invention relates generally to a method for improving a failure mode diagnostic of an electric power steering (EPS) system and, more particularly, to a method for improving a failure mode diagnostic of an EPS system by overcoming a motor electric lock condition as a result of position sensor signal corruption or other motor faults.
2. Discussion of the Related Art
Most modern electric power steering systems for vehicles employ a brushless permanent magnet synchronous motor coupled to the steering gear to generate a steering assist torque. A hand-wheel torque sensor is used to sense driver torque demand and a position sensor is used to detect the absolute motor position and the steering angle. An electronic control unit (ECU) controls the magnitude and phase of the motor currents to be in synchronism with the rotor position to control the motor torque using rotor absolute position and the driver hand-wheel torque. A corrupted motor position sensor signal could lead to reduced torque assist, or even a negative torque assist, that could result in an electric lock of the steering system.
For some applications, the detection of sensor signal degradation requires rotation of the motor. However, during an electric lock condition, it is generally not possible for the motor to rotate and hence the sensor signal degradation cannot easily be diagnosed to revert the system to a manual steering mode.
Known techniques for addressing this issue have compared the voltage levels of the motor position sensor signals, where the sensor can be a resolver or an optical encoder, to preset levels to determine any signal degradation, and disable the motor torque in the event of sensor malfunction.